$t = amp;2011-07-07
This line wont work as is, because PHP doesnt know what 'amp' means.
$t = "amp;2011-07-07";
Now PHP knows what that is - it's a string. Or at least, it's a potential string.
PHP is loosely typed for basic data types; so...
$t = "1";
echo $t; //2
PHP took my string, 1, saw that i wanted to do a mathematical operation for it, implicitly converted it to an numerical value, added 1 to it, and then output it as a string again.